The ultimate purpose of a communication system is to transmit information from an information source to a destination over a communication channel. In wireless communication systems, noise and multipath fading cause bit errors to occur during transmission. A variety of error control techniques are used to combat interference and reduce bit errors. Most error control techniques introduce controlled redundancy into the information transmitted over the communication channel that can be used at the destination to detect and possibly correct bit errors that may occur during transmission. Two commonly used error control techniques are automatic repeat request (ARQ) and forward error correction (FEC).
The basic principle underlying ARQ is to add redundant bits or check bits to a message that allows detection of errors at the receiver. If the receiver detects errors in the received message, the receiver can request a repeat transmission of the message, either by sending an explicit request for retransmission or by not acknowledging receipt of the message. Retransmissions are typically at the same data transmission rate as the original transmission. The number of retransmissions may be limited to a predetermined maximum number. ARQ is simple and achieves reasonable throughput when the error rate is low. Throughput diminishes, however, as the error rate increases because of the need to resend data.
FEC uses error-correcting codes to combat errors by adding redundancy to information before it is transmitted enabling the receiver to detect and correct most errors that occur during transmission. A receiver with knowledge of the error correcting code can detect and correct most bit errors. Examples of FEC codes include block codes, convolutional codes, and turbo codes.
Hybrid ARQ (HARQ) is another error control technique that combines ARQ and FEC. HARQ is generally adopted for high-speed packet data (HSPD) channels to further ensure robustness against link adaptation errors. Using HARQ, messages are coded twice using an inner code and an outer code. The outer code may, for example, comprise a CRC code that is appended to the information bits prior to transmission to form a protected message. The protected message is then coded using FEC, for example, a convolutional code or turbo code. Both the information bits and CRC bits are coded. The coded message is then transmitted to a receiving terminal, which decodes the message and performs a CRC check. If the number of errors in the message is within the capabilities of the error correction code, the errors will be corrected without the need for retransmission. Only if the number of errors in the message exceeds the capabilities of the error correcting code will retransmission be requested.
When ARQ and HARQ are employed on a reverse link channel, the mobile station needs to take into account the possibility that the original frame may need to be retransmitted at the same rate if the original transmission fails. Thus, the mobile station will normally reserve some power, so that it will be able to maintain its current data transmission rate even if channel conditions worsen. The reserved power is generally referred to as power headroom. If the power headroom is too small and conditions significantly worsen, the mobile station may not be able to retransmit bad frames as required by the ARQ and HARQ. On the other hand, if too much power is reserved, the average data transmission rate of the mobile station will be reduced.